1. Field of the Invention
This invention relates to devices and methods for the collection of solar energy using a photovoltaic solar cell.
2. Back ground and Description of the State of the Art
FIGS. 1A-1B illustrates a conventional construction for photovoltaic (PV) cells, e.g., a solar cell, which converts light energy to electricity. In this example, the cell 1 is generally square-like with an upper surface 2 corresponding to a sun-receiving or sun-facing side 2 of the solar cell 1. The opposing surface 3 faces away from the direction of sunlight. In this example, the solar cell 1 has an n-type doped silicon layer and p-type doped silicon layer separated by a junction layer. The sun-facing side 2 has a surface area made up of the PV material, e.g., n-type doped silicon, and contacts 5 for conducting a current when the PV material receives light energy. The contacts 5 are in the shape of lengthwise conducting lines 5a and two central buses 5b for collecting charge from the conducting lines 5a. The central buses 5b, and the corresponding electrode 5c for collecting a positive charge, are electrically connected to adjacent solar cells (preferably, a parallel connection) to form a solar cell module. Several modules connected together form a solar cell panel or solar panel.
A photovoltaic power system may have a single solar cell module or panel, or multiple modules/panels connected by combinations of series and parallel circuits as a photovoltaic array, or solar array. In the case of a single module system producing AC power output, the solar cell module may be connected to an inverter or load through a junction box that incorporates a fuse to protect the photovoltaic module if back feeding from a power utility or a battery might occur. Solar cell modules may be configured either with a frame or without a frame. A frameless solar cell module is generally referred to as a laminate. Examples of power systems having interconnected solar modules or panels may be found in U.S. Pub. No. 2003/0111103.
In recent years efforts are being made to develop thin-film PV material. Some such types of material are a-Si, CdTe, and CIGS. At present, these solutions have an efficiency rating of about 10% in commercially available panels (in laboratory conditions CIGS has been shown to be up to about 20% efficient). some of these thin-film technologies suffer from degradation and loss of efficiency over time. Presently, single crystalline PV, and to some extent poly-Si (although not as efficient), offer highest efficiency as a long-term proven technology.
Concentrated photovoltaics (CPV) utilize lenses or mirrors to focus, or concentrate solar energy onto PV material. Low concentrators, e.g., 2× concentrators, can require no tracking or movement (e.g., linear Winston collectors). These concentrators often times need to be orientated in an East-to-West orientation and tilted towards the celestial equator which may not be easily accomplished given the available mounting locations. Additionally, these concentrators can be expensive to manufacture because they use non-standard shapes for the lenses, e.g., Winston collectors. High concentrators need active moving parts in two dimensions and active cooling of the PV cells.
A “concentrator” is intended to mean a device that concentrates reflected and/or refracted light for the purposes of increasing the solar flux onto PV material. Unless otherwise noted, when a “lens” or “lenses” is/are referred to, it carries the same meaning as a concentrator, i.e., intended to focus or concentrate solar energy. A “linear”, “one-dimensional” or “1-D” concentrator is intended to mean a concentrator configured for focusing light in only one-dimensions. One example of a 1-D concentrator is a rod lens. A “two-dimensional” or “2-D” concentrator is intended to focus light in two dimensions. A spherical lens is one example of a 2-D concentrator. A 1-D concentrator has a “line of focus”, which is intended to mean the line or strip of focused light over the length of the concentrator. A line of focus for a 1-D concentrator, oriented east to west, changes during the course of the year. Thus, for a 1D concentrator at the Earth's equator and pointed at the celestial equator at equinox, the line of focus is parallel to the lens. At the periods of solstice the line of focus is distanced farthest from this condition. Throughout the discussion, reference may sometimes be made to a solar panel, module or solar collection system. One of ordinary skill would appreciate that the terms ‘solar panel’ or ‘solar module’ do not necessarily limit the scope of the disclosure. Unless otherwise apparent, the disclosure applies to any solar collection device that utilizes concentrators to focus light onto PV material in accordance with the foregoing objectives.
A “tracking panel” is intended to mean a panel that is maintained to face the sun such that light rays are always orientated normal to the panel surface. A tracking panel may be rotated so that solar rays are always directed normal to the surfaces having PV material. Accordingly, the effective area, AEFF for a tracking panel is intended to always be equal to the corresponding surface area APV of the panel. For a panel that does not track the sun's motion AEFF<APV whenever the sun's apparent position changes from the position that the panel faces. For a fully tracked panel (as opposed to a partially tracked panel, e.g., partial tracking when rotation about only one axis as the sun changes its position in the sky) the acceptance angle is of lesser or no importance to a design because the panel is being tracked. For a partially tracked panel, e.g., when linear concentrators rotate about their longitudinal axes, the acceptance angle requirements reduce to only one dimension: the longitudinal dimension. Therefore, a linear concentrator is designed such that it provides a very large acceptance angle along its longitudinal direction so that there would be no need to move them, or the panel, in that direction.
For solar panels that use concentrators, “acceptance angle” means the angle beyond which not all focused or collected light is received on PV material. A CPV panel that does not track the sun but still collects sun light from morning to evening throughout the year (hereafter refer to as panels that increase the acceptance angle) does this in a few ways. It can be made of smaller units, such as linear Winston collectors, where the acceptance angle is designed into each collector. With such a design the CPV panel is oriented usually in an East-to-West direction and tilted towards the celestial equator. A CPV panel can also collect sunlight without tracking by incorporating moving parts within the panel, while the panel remains stationary, such as moving mirrors that focus light onto their corresponding focal points where a PV cell is positioned.
A solar panel's “collection plane” is intended to refer to the plane where the PV material is generally located to receive solar flux. For example, a two-axis tracking solar panel is configured to be rotated about two orthogonal axes that lie within the collection plane (x and y axes depicted in FIG. 8A). With reference to FIG. 7 the vector Pn extends normal to the collection plane for the panel shown.